The present invention relates generally to inductive plasma processors with RF plasma excitation coils and, more particularly, to such a processor with a coil including a planar winding segment that is electrically connected to a planar turn, wherein the segment is stacked vertically relative to a portion of the planar turn. Another aspect of the invention relates to a processor including a coil with a winding having a planar turn having ends that are in a first plane and connected with turns or partial turns having ends in a second plane wherein the coil is driven from RF excitation terminals that are spaced from the first and second planes and the turn ends are connected to (1) each other and/or (2) the excitation terminals by connecting structures that extend smoothly and gradually, without sharp bends, between opposite ends of the connection structure.
One type of processor for treating workpieces with an RF plasma in a vacuum chamber includes a coil connected to be responsive to an RF source by leads extending vertically between terminals located in a housing above the coil. The coil, which is usually planar or spherical or dome shaped, is driven by the RF source to produce electromagnetic fields that excite ionizable gas in the chamber to a plasma. The leads connecting the coil to the excitation source intersect terminals of the coil at right angles. Usually the coil is on or adjacent to a dielectric window that extends in a direction generally parallel to a planar horizontally extending surface of the processed workpiece. The excited plasma interacts with the workpiece in the chamber to etch the workpiece or to deposit material on it. The workpiece is typically a semiconductor wafer having a planar circular surface or a solid dielectric plate, e.g., a rectangular glass substrate used in flat panel displays, or a metal plate.
Ogle, U.S. Pat. No. 4,948,458 discloses a multi-turn spiral planar coil for achieving the above results. The spiral, which is generally of the Archimedes type, extends radially and circumferentially between its interior and exterior terminals connected to the RF source via an impedance matching network. Such coils produce oscillating RF fields having magnetic and electric field components that penetrate through a dielectric window to excite electrons and ions in a portion of the plasma chamber close to the window. The spatial distribution of the magnetic field in the plasma portion close to the window is a function of the sum of individual magnetic field components produced by the current at each point of the coils. The inductive component of the electric field is produced by the time varying magnetic field, while the capacitive component of the electric field is produced by the RF voltage in the coils. The inductive electric field is azimuthal while the capacitive electric field is vertical to the workpiece. The current and voltage differ at different points because of transmission line effects of the coil at the frequency of the RF source.
For spiral designs as disclosed by and based on the Ogle ""458 patent, the RF currents in the spiral coil are distributed to produce a toroidal shaped electric field resulting in a toroidal plasma close to the window, which is where power is absorbed by the gas to excite the gas to a plasma. The toroidal shaped magnetic field is accompanied by a ring shaped electric field which generates a toroidal shaped plasma distribution. At low pressures, in the 1.0 to 10 mTorr range, diffusion of the plasma from the toroidal shaped region where plasma density is peaked tends to smear out plasma non-uniformity and increases plasma density in the chamber center just above the center of the workpiece. However, the diffusion alone generally can not sufficiently compensate plasma losses to the chamber walls and plasma density around the workpiece periphery can not be changed independently. At intermediate pressure ranges, in the 10 to 100 mTorr range, gas phase collisions of electrons, ions, and neutrals in the plasma further prevent substantial diffusion of the plasma charged particles from the toroidal region. As a result, there is a relatively high plasma density in a ring like region of the workpiece but low plasma densities in the center and peripheral workpiece portions.
These different operating conditions result in substantially large plasma flux (i.e., plasma density) variations between inside the toroid and outside the toroid, as well as at different azimuthal angles with respect to a center line of the chamber that is at right angles to the plane of the workpiece holder (i.e., chamber axis). These plasma flux variations result in a substantial standard deviation, i.e., in excess of six percent, of the plasma flux incident on the workpiece. The substantial standard deviation of the plasma flux incident on the workpiece has a tendency to cause non-uniform workpiece processing, i.e., different portions of the workpiece are etched to different extents and/or have different amounts of materials deposited on them.
Many arrangements directed to improving the uniformity of the plasma density incident on a workpiece have concentrated on geometric principles, usually concerning coil geometry. See, e.g., U.S. Pat. Nos. 5,304,279; 5,277,751; 5,226,967; 5,368,710; 5,800,619; 5,401,350; 5,558,722, 5,759,280, 5,795,429, 5,847,074 and 6,028,395. However, these coils have generally been designed to provide improved radial plasma flux uniformity and to a large extent have ignored azimuthal plasma flux uniformity or azimuthal symmetry.
Our commonly assigned U.S. Pat. No. 6,164,241 entitled xe2x80x9cMultiple Coil Antenna for Inductively-Coupled Plasma Generation Systems,xe2x80x9d discloses another coil including two concentric electrically parallel windings each having first and second terminals, which can be considered input and output terminals of each winding. Each first terminal is connected via a first series capacitor to an output terminal of a matching network driven by an RF power source. Each second terminal is connected via a second series capacitor to a common ground terminal of the matching network and RF source. Each winding can include a single winding or multiple windings that extend circumferenfially and radially in a spiral-like manner relative to a common axis of the two windings. Each winding is planar or three-dimensional (i.e., spherical or dome-shaped) or separate windings of a single winding can be stacked relative to each other to augment the amount of electromagnetic fields coupled by a particular winding to the plasma.
Holland et al, U.S. Pat. No. 6,028,395, discloses a coil including plural electrically parallel windings. Peripheral parts of the windings are stacked vertically with respect to each other and a dielectric window separating the coil from the vacuum chamber interior. The stacked coil segments are arranged so that the electromagnetic fields resulting from current flowing in parallel through the two segments is additive, to assist in maintaining relatively uniform electromagnetic fields in the chamber and a relatively uniform plasma density on the workpiece.
The parallel connections of the stacked coil portions are established by struts that extend substantially perpendicular to the two parallel, stacked coil portions. Adverse effects may occur as a result of the leads being connected perpendicular to the coil terminals. In particular, we have found that the struts and leads seem to perturb the electromagnetic fields generated by the coil and stacked coil segments particularly around the region where the leads and coil terminals are connected. In addition the struts and leads have a tendency to produce in the coil relatively large standing wave variations which usually cause a non uniform plasma to be incident on the workpiece.
It is accordingly an object of the present invention to provide a new and improved coil for a vacuum plasma processor.
An additional object of the present invention is to provide a new and improved coil for a vacuum plasma processor wherein the plasma density incident on a workpiece of the processor has relatively high azimuthal and radial uniformity.
A further object of the invention is to provide a new and improved connection structure between RF excitation terminals driving a plasma excitation coil of a vacuum plasma processor and terminals of the coil.
Another object of the invention is to provide a new and improved connection structure between portions of a plasma excitation coil that are in different planes relative to a dielectric window of the processor.
According to one aspect of the invention, a vacuum plasma workpiece processor multi-turn plasma excitation coil which is arranged to be positioned above a window of a vacuum chamber of the processor has at least one substantially planar turn and a segment stacked with a portion of the planar turn. The stacked segment is spaced from the planar turn by a distance different from the spacing between the planar turn winding and the roof of the chamber interior (typically the window thickness) and is connected in series with the planar turn so the same current flows in the same direction through the planar turn and the stacked segment. The stacked segment is mainly used to increase the inductive RF coupling to a particular region of the plasma to improve the azimuthal, plasma uniformity and correct azimuthal asymmetries due to the chamber and coil not being perfectly symmetric. The position of the stacked segment, the arc length of the stacked segment, and spacing between the stacked segment and the planar turn are preferably selected for each chamber and/or coil configuration to optimize the RF coupling to a particular region of plasma.
The stacked segment preferably includes first and second terminals for series connection to the planar turn. The stacked segment forms an additional, extended partial turn having opposite first and second terminals connected to the planar turn. In one embodiment, a metal lead which establishes the connection between the second terminal of the stacked segment and a first end of the planar turn includes an interconnection loop. First and second ends of the loop are respectively connected to the second terminal of the stacked portion and the first end of the planar turn. The loop is bent gradually and smoothly without sharp bends such that it does not substantially perturb the electromagnetic fields produced by the stacked segment and the planar turn.
According to another aspect of the invention, a first metal connection structure has first and second ends respectively connected to an input terminal of the coil and an output of a matching network. A second metal connection structure has a first end connected to the coil output terminal and a second end connected to a termination capacitor. The first and second metal connection structures extend gradually and smoothly without sharp bends such that electromagnetic fields produced by them are constructively superimposed on the main fields produced by the coil (as well the stacked segment if applicable).
In accordance with a further aspect of the invention, a metal connection structure having first and second ends respectively connected to a first portion of the planar turn and an end of the stacked segment is arranged so it extends gradually and smoothly without sharp bends between the first and second ends thereof.
Preferably, the stacked segment is located adjacent an interconnection gap between ends of a planar winding. The stacked segment extends in both directions from the gap to compensate low RF coupling from the gap to the plasma.
In one embodiment, the metal connection structure includes (1) a first part that loops from the first end of the planar turn away from the gap so it extends in a direction away from the gap to a point that is farther from the gap than a first end of the stacked segment and (2) a second part that loops back from the point to the first end of the stacked segment. The first end of the metal connection structure preferably extends tangentially relative to the first end of the stacked segment. The stacked segment includes a second end that defines one of the coil terminals connected to be responsive to current from the RF source.
According to an additional aspect of the invention, a planar turn includes first and second end portions that are spatially close to each other and are spaced by a gap from each other so current flows around the remainder of the planar turn between the first and second end portions. One of the end portions is connected by a radially and circumferentially extending conductive strap to an adjacent turn of the winding. The stacked segment extends across the gap so first and second ends of the stacked segment are on opposite sides of the gap.
According to a first embodiment, first and second ends of the stacked segment are displaced by approximately equal angles from the interconnection gap between adjacent planar turns. The connecting structure is arranged so current first flows back in the direction opposite to the direction of current flow in the planar turn, then turns direction gradually following the gradually bent connection structure, and eventually flows in the same direction as the original current flow in the planar turn.
According to a second embodiment, the first and seconds ends of the stacked segment are arranged so the second end is displaced circumferentially across the gap interconnecting adjacent turns and extends substantially greater than the angular displacement of the gap to compensate low RF coupling from the gap region to the plasma. Preferably, the stacked segment has the second end of the stacked segment vertically overlaying the first end of the planar turn. The second end of the stacked segment can be connected to the first end of the planar turn via a short, straight connection so current continues to flow in the same direction in the planar turn.
The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed descriptions of several specific embodiments thereof, especially when taken in conjunction with the accompanying drawings.